This thesis aims to contribute to the understanding of Witold Lutoslawski's concept of akcja ("action" or "plot"). After an Introduction outlining the study's motivations and presenting a critical survey of the Lutoslawski literature on akcja and related topics, its investigation proceeds through three broad phases which seek to make independently valuable contributions to their respective areas while building towards an elucidation of akcja. First, unpublished Lutoslawski lectures from the 1960s that reveal traces of his poetics of plot are examined against the backdrop of his polemical views and the influences on his creative approach. Second, theories of plot, narrative and musical narrativity are utilized to devise a strategy for the analysis of instances of akcja; as part of this process, ideas on musical narrativity are developed in relation to such issues as plot and emplotment, story and discourse, meaning and metaphor, and narrativity in twentieth-century music. Third, that strategy is applied in an analysis of Lutoslawski's 1968 composition Livre pour orchestre, a piece - part symphony, part modernist "Livre" - in which the issue of narrativity itself becomes an aspect of the musical narrative. The study's Afterword then presents a sketch for a new theory of musical narrativity, plus analyses of passages from Lutoslawski pieces composed during different periods in his career, thereby demonstrating the centrality of akcja to his music and, in turn, to the development of an enhanced appreciation of Lutoslawski's creative achievements.